During touch-down and ultrasonic bonding stages of wirebonding, high stresses are applied onto a bond pad so that a good joint between a bonding wire and the bond pad may be achieved. However, these high stresses may cause pad damage, silicon cratering, and aluminium splash where the thickness of the aluminum at the center of the pad is squeezed to the sides. These factors may result in poor bond quality.
With the trend towards miniaturisation in the electronics industry, the bonding wire is getting thinner, the bond pad space is getting narrower and the bond pad size is being shrunk. This means that a larger percentage of the bond pad area is experiencing a large stress. Further, the trend towards copper low-K (e.g. low dielectric constant) and ultra low-K also poses a challenge as the materials are soft and brittle. Wirebonding has been found to cause cracking and delamination in the low-K layers. The above mentioned problems may be further worsened with copper wire bonding rather than gold wire bonding as copper is harder than gold. Mechanical simulation shows that the stresses on the pad are much higher with copper wire than with gold wire.
These failures due to high stresses can be reduced if bonding parameters such as the free air ball shape, touchdown force and durations as well as ultrasonic energy and duration are optimized with knowledge of the stress state under the bond pad. Capillary shape also plays a role in determining the shape and the strength of the bond and the stress induced on the bond pad. Capillary design can also be optimized taking into account the stresses induced on the bond pad.
Measuring and mapping the stress distribution under the bond pad and correlating these to the failure mechanisms of wirebonding will allow easier determination of process parameters and improved capillary designs such that failures such as silicon cratering, aluminium splash and low-K cracking can be avoided and more robust wirebonds can be formed. Conventional methods to measure the stress distribution under the bond pad may involve mechanical simulation for extrapolation of stress from the measured regions to the unmeasured regions.
One conventional method is to place four n-type piezoresistive sensors arranged in a Wheatstone bridge configuration at the corners of bond pads [1-4]. All four sensors are aligned along the same crystallographic direction. With this configuration, there is a single output signal which corresponds to a stress component σxy. The stress component σxy is a shear stress. The subscript x of the stress component σxy represents the direction normal to the surface upon which the stress acts upon. The subscript y of the stress component σxy represents the direction of the stress itself. Finite element modeling is then used to correlate σxy to another stress component σz. The stress component σz is a normal stress. The subscript z of the stress component σz represents the surface on which the stress acts. However, this method determines only one component of stress which is the shear stress outside the pads. This method does not take stress measurement under the pad where failure usually occurs. Further, one assumption made using this method is that all four resistors have the same resistance which is highly unlikely given current fabrication methods.
Another conventional method includes placing single line n-type sensor across the center of bond pad and serpentine n-type sensor over the entire bond pad area [5]. However, this method measures bond force and shows that bond force has a linear relationship with resistance change. This method does not measure stress under the bond pad.